1. Field of the Invention
This invention relates to a metallocene catalyst component useful in preparing isotactic polyolefins. The catalyst component consists of a bridged metallocene in which one of the cyclopentadienyl rings is substituted in a different manner from the other ring. The invention further includes a process of preparing isotactic polyolefins that comprises the use of one or more of the disclosed catalyst components and also a process for preparing the catalyst components.
2. Description of the Prior Art
The present invention provides a catalyst and process for polymerizing olefins having three or more carbon atoms to produce a polymer with a isotactic stereochemical configuration. The catalyst and process are particularly useful in polymerizing propylene to form isotactic polypropylene. The isotactic structure is typically described as having the methyl groups attached to the tertiary carbon atoms of successive monomeric units on the same side of a hypothetical plane through the main chain of the polymer, e.g., the methyl groups are all above or all below the plane. Using the Fischer projection formula, the stereochemical sequence of isotactic polypropylene is described as follows: ##STR1##
Another way of describing the structure is through the use of NMR spectroscopy. Bovey's NMR nomenclature for an isotactic pentad is . . . mmmm . . . with each "m" representing a "meso" dyad or successive methyl groups on the same side in the plane. As known in the art, any deviation or inversion in the structure of the chain lowers the degree of isotacticity and crystallinity of the polymer.
In contrast to the isotactic structure, syndiotactic polymers are those in which the methyl groups attached to the tertiary carbon atoms of successive monomeric units in the chain lie on alternate sides of the plane of the polymer. Using the Fischer projection formula, the structure of a syndiotactic polymer is designated as: ##STR2##
In NMR nomenclature, this pentad is described as . . . rrrr . . . in which each "r" represents a "racemic" dyad, i.e., successive methyl group on alternate sides of the plane. The percentage of r dyads in the chain determines the degree of syndiotacticity of the polymer. Syndiotactic polymers are crystalline and, like the isotactic polymers, are insoluble in xylene. This crystallinity distinguishes both syndiotactic and isotactic polymers from an atactic polymer that is soluble in xylene. Atactic polymer exhibits no regular order of repeating unit configurations in the polymer chain and forms essentially a waxy product.
While it is possible for a catalyst to produce all three types of polymer, it is desirable for a catalyst to produce predominantly isotactic or syndiotactic polymer with very little atactic polymer. Catalysts that produce isotactic polyolefins are disclosed in copending U.S. patent application Ser. No. 317,089 filed Feb. 28, 1989, and U.S. Pat. Nos. 4,794,096 and 4,975,403. This application and these patents disclosed chiral, stereorigid metallocene catalyst components that are used in the polymerization of olefins to form isotactic polymers and are especially useful in the preparation of a highly isotactic polypropylene. Catalysts that produce syndiotactic polyolefins are disclosed in U.S. Pat. No. 4,892,851. This patent discloses stereorigid metallocene catalyst components having sterically different cyclopentadienyl ligands used in the polymerization of olefins to form syndiotactic polymers and are especially useful in the preparation of highly syndiotactic polypropylene. Catalysts that produce hemi-isotactic polyolefins are disclosed in U.S. Pat. No. 5,036,034. This patent disclosed stereorigid metallocene catalyst components having a lack of bilateral symmetry used in the polymerization of olefins to form hemi-isotactic polymers and are especially useful in the preparation of hemi-isotactic polypropylene.
The isospecific metallocene catalyst components of the prior art did not exhibit bilateral symmetry of the cyclopentadienyl ligands. The syndiospecific metallocene catalyst components have bilateral symmetry or pseudo-bilateral symmetry of the cyclopentadienyl ligands. The hemi-isospecific catalyst components have bilateral symmetry or pseudo-bilateral symmetry in only one cyclopentadienyl ligand. The present invention, however, provides a different class of metallocene catalyst components that are useful in forming a catalyst for polymerization to produce isotactic polyolefins and, more particularly, isotactic polypropylene. The isospecific metallocene catalyst components of the present invention have bilateral symmetry or pseudo-bilateral symmetry in only one cyclopentadienyl ligand and bulky substituents on the other cyclopentadienyl ligand.